Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A28: Biofluids: Motility in Newtonian and Non-Newtonian Fluids |
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Chair: F. Zeynep Temel, Brown University Room: 309 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A28.00001: Motility modes of the parasite \textit{Trypanosoma brucei} Fatma Zeynep Temel, Zijie Qu, Michael McAllaster, Christopher de Graffenried, Kenneth Breuer The parasitic single-celled protozoan \textit{Trypanosoma brucei} causes African Sleeping Sickness, which is a fatal disease in humans and animals that threatens more than 60 million people in 36 African countries. Cell motility plays a critical role in the developmental phases and dissemination of the parasite. Unlike many other motile cells such as bacteria \textit{Escherichia coli} or \textit{Caulobacter crescentus}, the flagellum of \textit{T. brucei} is attached along the length of its awl-like body, producing a unique mode of motility that is not fully understood or characterized. Here, we report on the motility of \textit{T. brucei}, which swims using its single flagellum employing both rotating and undulating propulsion modes. We tracked cells in real-time in three dimensions using fluorescent microscopy. Data obtained from experiments using both short-term tracking within the field of view and long-term tracking using a tracking microscope were analyzed. Motility modes and swimming speed were analyzed as functions of cell size, rotation rate and undulation pattern. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A28.00002: The fluid dynamics of the ciliate \textit{Pseudotontonia} sp. jumping by ''tail'' contraction Houshuo Jiang, Brad Gemmell, Edward Buskey The marine planktonic ciliate \textit{Pseudotontonia} sp. ($\sim$ 80 $\mu $m in cell size) possesses two sets of propulsive machinery: (1) an anteriorly located ciliary band that beats to let the cell swim backward, and (2) a long, contractile appendage (i.e. the `tail') that at times contracts rapidly to pull the cell body backward, resulting in the tail contraction and body jumping motion being oppositely directed inwards towards the same location. We use high-speed microscale imaging and micro-particle image velocimetry techniques to measure the ciliate swimming and jumping kinematics and imposed flow fields. We show that the cilia-propelled swimming achieves a sustained swimming speed $\sim$ 10 mm s$^{-1}$ that can last more than 100 ms. The swimming imposed flow conforms to the steady stresslet flow field that decays spatially at $r^{-2}$. On the other hand, the tail contraction causes the cell to jump at a peak speed $\sim$ 55 mm s$^{-1}$ and cover a jumping distance 2-4 cell lengths within $\sim$ 12 ms jumping time. The jumping imposed flow fits quite well to the unsteady impulsive stokeslet flow field that decays spatially at $r^{-3}$. Based on the measured jumping kinematics, we develop a fluid dynamics model to explain the thrust generation due to the tail contraction. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A28.00003: Investigation of the swimming mechanics of \textit{Schistosoma cercariae} and its role in disease transmission Deepak Krishnamurthy, Arjun Bhargava, Georgios Katsikis, Manu Prakash Schistosomiasis is a Neglected Tropical Disease responsible for the deaths of an estimated 200,000 people annually. Human infection occurs when the infectious forms of the worm known as cercariae swim through freshwater, detect humans and penetrate the skin. Cercarial swimming is a bottleneck in disease transmission since cercariae have finite energy reserves, hence motivating studies of their swimming mechanics. Here we build on earlier studies which revealed the existence of two swimming modes: the tail-first and head-first modes. Of these the former was shown to display a novel symmetry breaking mechanism enabling locomotion at low Reynolds numbers. Here we propose simple models for the two swimming modes based on a three-link swimmer geometry. Using local slender-body-theory, we calculate the swimming gait for these model swimmers and compare with experiments, both on live cercariae and on scaled-up robotic swimmers. We use data from these experiments and the models to calculate the energy expended while swimming in the two modes. This along with long-time tracking of swimming cercariae in a lab setting allows estimation of the decrease in activity of the swimmer as a function of time which is an important factor in cercarial infectivity. Finally, we consider, through experiments and theoretical models, the effects of gravity since cercariae are negatively buoyant and sink in the water column while not swimming. This sinking affects cercarial spatial distribution which is important from a disease perspective. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A28.00004: Caulobacter crescentus exploits its helical cell body to swim efficiently Bin Liu, Marcos Mendoza, Joanna Valenzuela How an organism gets its shape remains an open question of fundamental science. In this study, we measure the 3D shape of a bacterium, Caulobacter crescentus, using a computational graphic technique for free-swimming microorganisms to analyze thousands of image frames of the same individual bacterium. Rather than having a crescent shape, the cell body of the organism is found to be twisted with a helical pitch angle around 45 degrees. Moreover, the detailed size and geometry of the cell body, matches the optimized cell body obtained by the slender body theory for swimming at fixed power. This result sheds new light on the shape evolution of microorganisms, and suggests that C. crescentus has adapted to its natural habitat of fresh-water lakes and streams, lacking nutrients. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A28.00005: Enhancement of flagellated bacterial motility in polymer solutions Wenyu Zhang, Sha Sha, Robert Pelcovits, Jay Tang Measurements of the swimming speed of many species of flagellated bacteria in polymer solutions have shown that with the addition of high molecular weight polymers, the speed initially increases as a function of the kinematic viscosity. It peaks at around 1.5-2 cP with typically 10-30{\%} higher values than in cell media without added polymers ($\sim$ 1 cP). Past the peak, the average speed gradually decreases as the solution becomes more viscous. Swimming motility persists until solution viscosity reaches 5-10 cP. Models have been proposed to account for this behavior, and the magnitude of the peak becomes a crucial test of theoretical predictions. The status of the field is complicated in light of a recent report (Martinez et al.,PNAS, 2014), stressing that low-molecular weight impurities account for the peaked speed-viscosity curves in some cases. We measured the swimming speed of a uni-flagellated bacterium, caulobacter crescentus, in solutions of a number of polymers of several different sizes. Our findings confirm the peaked speed-viscosity curve, only as the molecular weight of the flexible polymers used surpassed $\sim$ 50,000 da. The threshold molecular weight required to augment swimming speed varies somewhat with the polymer species, but it generally corresponds to radius of gyration over tens of nanometers. This general feature is consistent with the model of Powers et al. (Physics of Fluid, 2009), predicting that nonlinear viscoelasticity of the fluid enhances swimming motility. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A28.00006: A fluid model for Helicobacter pylori Shang-Yik Reigh, Eric Lauga Swimming microorganisms and self-propelled nanomotors are often found in confined environments. The bacterium \textit{Helicobacter pylori} survives in the acidic environment of the human stomach and is able to penetrate gel-like mucus layers and cause infections by locally changing the rheological properties of the mucus from gel-like to solution-like. In this talk we propose an analytical model for the locomotion of \textit{Helicobacter pylori} as a confined spherical squirmer which generates its own confinement. We solve analytically the flow field around the swimmer, and derive the swimming speed and energetics. The role of the boundary condition in the outer wall is discussed. An extension of our model is also proposed for other biological and chemical swimmers. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A28.00007: Buckling Instabilities and Complex Dynamics in a Model of Uniflagellar Bacterial Locomotion Frank Nguyen, Michael Graham Locomotion of microorganisms at low Reynolds number is a long studied problem. Of particular interest are organisms using a single flagellum to undergo a wide range of motions: pushing, pulling, and tumbling or flicking. Recent experiments have connected the stability of the hook protein, connecting cell motor and flagellum, to deviations from typical straight swimming trajectories. We seek physical explanations to these phenomena by developing a computationally inexpensive, rigid-body dynamic model of a uniflagellated organism with a flexible hook connection that captures the fundamental dynamics, kinematics, and configurations. Furthermore, the model addresses the effects of hook loading and geometry on the stability of the system. Simulations with low hook flexibility produce the classic straight trajectory, but a large flexibility produces helical trajectories, leading to directional changes when coupled with transient hook stiffening. Minima for critical flexibilities are found in certain subsets of parameter space, implying preferred geometries for certain swimming dynamics. The model verifies proposed mechanisms for swimming in various modes and highlights the role of flexibility in the biology of real organisms and the engineering of artificial microswimmers. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A28.00008: MOVED TO M26.003 |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A28.00009: Dynamics of Buckling of an Elastic filament in a Viscous fluid Moumita Dasgupta, Arshad Kudrolli We study the buckling of an elastic filament when immersed in a Newtonian fluid as it undergoes a uniaxial compression. Although there have been investigations of buckling of semi-flexible filaments in complex materials including locomotion of microorganisms, in cytoskeleton of microtubules and helical plant roots, there is a gap in the understanding of the dynamics of buckling instability for the simpler Newtonian case. Therefore, we investigated the growth of buckled modes of an elastic ribbon under various compression rates which buckles into configurations which depend of the relative magnitude of the elastic and viscous forces. At low compression rates, the ribbon buckles quasi-statically to form the standard one-mode shape in agreement with the fundamental Euler buckling mode. As the compression rate is increased, the ribbon undergoes systematic increase in the number of modes at onset. In all cases, the ribbon relaxes after compression stops to the fundamental Euler mode. We will discuss the fits to the shape in terms of sums of Euler modes as well as Fourier modes, and their growth and decay. Finally, the effect of the fluid viscosity on the evolution of the buckled mode will be discussed. [Preview Abstract] |
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